Electrodeposition of tin

ABSTRACT

A method for the electroplating of steel strip in a tin plating bath utilizing therein a brightening agent comprising ethoxylated-naphthol sulfonic acid, wherein the ethylene oxide to naphthol ratio is 6 or 7:1.

United States Patent [191 Johnson Jan. 14, 1975 ELECTRODEPOSITION OF TIN [75] Inventor: William R. Johnson, Penn [56] References cued Township, Westmoreland County, UNITED STATES PATENTS Pa. 2,457,tg 12/1948 Hoffman 204/54 R 73 A :U'tdStt St lC t' sslgnee g i 5 ee orpora Primary Examiner-G. L. Kaplan [221 F 1 M 25 1 974 Attorney, Agent, or FirmArthur J. Greif i ed: at. [21] Appl. No.: 454,057 ABSTRACT A method for the electroplating of steel strip in a tin plating bath utilizing therein a brightening agent comg i; prising ethoxylated-naphthol sulfonic acid, wherein the ethylene oxide to naphthol ratio is 6 or 7:1. [58] Field of Search 204/54 R, 43 S, 44, 120,

204/123 4 Claims, 1 Drawing Figure PATENTEDJANI 4:975

313% Q\ Q .w v

m? 5 m vmew Q ow t 5 MT "6am I SW0 EW SWEM 3 1 S mmam ELECTRODEPOSITION OF TIN BACKGROUND OF THE INVENTION brightening agents in the plating bath.

The electrodeposition of tin can be effected in a variety of acid plating electrolytes. However, all such plating processes in commercial use require at least three components: (1) an acid source, (2) a soluble tin salt and (3) one or more organic addition agents for the production of smooth, adherent and lustrous deposits of tin. With respect to the latter, various polyalkylene oxides have been known to be effective. Examples of such brightening agents are shown in (1.8. Pat. No. 2,457,152. Of particular benefit are the agents formed by reacting 1 mol of naphthol with from about 3 to 15 mols of ethylene oxide, i.e. wherein n 3 to 15 in the formula C H O-(-C H O-)-, H. Although they represent a considerable advance, these latter agents suffer from a number of disadvantages, especially when employed in commercial high speed plating lines. The shorter chain varieties, i.e. those in which n varies from about 3 to 9 are only limitedly soluble in conventional acid electrolytes, such as H SO or phenolsulfonic acid (PSA). As a result of such limited solubility, tarry products are formed in the plating bath, which deposit on the conductor rolls, the plating tanks and electrodeposited coating. The longer chain adducts, on the other hand, are substantially totally soluble. However, when employed in effective amounts, these longer chain adducts: (a) produce excessive amounts of foam; which decreases the conductivity of the electrolyte, is-difficult to control and therefore requires the addition of expensive antifoaming agents, and (b) exhibit a more limited plating range; thus, requiring the use of higher current It is yet another object of this invention to provide a completely soluble brightening agent with a desirably long shelf life, such that its high solubility is retained after long periods of storage.

These and other objects and advantages of the instant invention will be more apparent from a reading of the following description taken together with the appended claims and the drawing in which:

The FIGURE is a graph illustrating the superior shelf life of the brightening agents of this invention.

The ethoxylated-naphthol (EN) addition agents of U.S. Pat. No. 2,457,152 (examples 22 and 23) were initially studied to determine their effectiveness in high speed commercial tin-plating lines. It may be seen from Table I below, that these adducts can be made completely soluble in acid plating baths by increasing the ethylene oxide chain length, i.e. wherein n a 10. However, while the solubility is enhanced by increasing the chain length, other difficulties arise. Thus, excessive amounts of foam are encountered. Small amounts of foam are generally desirable to control the misting associated with acid plating. However, larger amounts of 5 foam will obviously create a containment problem.

Moreover, as noted above, the conductivity of the electrolyte is decreased by such excessive amounts of foam. Equally important, as the chain length is increased. the permissible plating range becomes narrower. That is,

the minimum current density required for the producthe longer chain length adducts would necessitateextensive modification of existing commercial plating facilities to accommodate such higher currents.

densities to achieve a bright deposit, i.e. one which is not cloudy in appearance.

It is therefore a principle object of this invention to provide an organic brightening agent which does not produce an excessive amount of foam; which exhibits a wide range of plating current densities and which is nevertheless completely soluble in the plating electrolyte.

It is a further object of this invention to provide a method for the production of such a brightening agent which avoids the production of undesirable byproducts.

When the EN adducts are sulfonated to produce ethoxylated-naphthol sulphonic acid (ENSA) a number of the above problems are overcome (see Table 11):.(1) the solubility is materially enhanced so that substantially complete solubility is achieved with ethylene oxide chain lengths equal to or greater than five, (2) desirable amounts of foam are achieved by employing ENSA adducts with chain lengths of from 5 to 7, and (3) a wider plating range is acheived for the adducts in which n 6 or 7.

TABLE II Plating Tests Minimum Current Density (Amps/ft) Solubility Test at 130F Dissolved Sample foaming Identification 4 gm/l 6 gmll characteristics H SO PSA ENSA-3 50 100 negligible 83 87 ENSA-S 75 75 light 100 96 ENSA-6 50 50 moderate lOO 99 at 6 gm/l ENSA-7 50 50 excessive 99 99 above 4 gm/l ENSAJO 75 excessive foam 99 99 at even 2 gm/l ENSA-l 5 excessive foam I I00 at even 2 gm/l heavy-powdery deposits formed Nevertheless, while the ENSA agents (especially those with chain lengths of to 7) were decidedly superior to the EN agents, additional problems were encountered. It was found that the solubility of the EN- SA-5 product in phenolsulfonic acid (PSA) was very sensitive to purity of the PSA. More importantly, regardless of acid purity, it was found that the solubility deteriorated with age, thereby posing a shelf-life problem. The FIGURE shows the amounts of tar-residue encountered in both phenosulfonic and sulfuric acid baths with ENSA-S and ENSA-6 brightening agents It may be seen that, in using the ENSA-S product, the amount of tar-residue encountered increases markedly after a storage period of about weeks, while no such increase is noted with the ENSA-6 product after a period of more than one year in duration.

Finally, even with the ENSA-6 and ENSA7 products, it was noted that if sulfonation was not conducted within a limited range of temperature and H 80. concentration, then undesirable by-products would form, which (a) had a deleterious effect on the appearance of the tin-plate and (b) led to increases in the amount of tarry residues encountered. It is therefore necessary that the ENSA-6 and ENSA-7 products of this invention be prepared in the following manner. The naphthol must initially be ethoxylated with ethylene oxide in the mole ratio of l to 6, or 1 to 7. With respect to their effect as brightening agents, the a and B varieties of naphthol may be employed interchageably. However, a naphthol should generally be employed, since the B variety is known to be toxic. The resulting adduct (i.e. EN-6 or EN7) is then sulfonated at a temperature of 5090C, preferably about 8085C, with sulfuric acid having a concentration of at least about 90%, preferably 92-98%. Within the prescribed range, the temperature employed should be approximately inverse to the concentration of the H 50 That is, temperature should be maintained in the upper end of its range when acid concentration approaches about 90%. However, as the concentration of the acid employed increases. temperature should then be maintained at correspondingly lower temperatures, so that the temperature should be about 50C when an acid concentration of about oleum is employed.

The ENSA6 and ENSA-7 brightening agents of this preferably 3 to 5 g/l, of ENSA6 or ENSA7 prepared in the manner prescribed above and (3) about 6 to 30 g/l, of free acid (calculated as H 50 The acid may be added either as H or phenolsulfonic acid. The temperature of the bath may vary from about 83 to 145F, preferably about to F. An electrolyzing current of at least about 50 amps/ft. is passed to the substrate as cathode in sufficient stages to form an electroplated tin coating of desired thickness.

I claim:

1. In the electroplating of ferrous substrates with a coating of tin, wherein the substrate, immersed in an electroplating bath, is made a cathode by the application of an electrolyzing current thereto, said plating bath being comprised of an acid electrolyte, a tin salt and at least one organic brightening agents dissolved therein,

the improvement which comprises employing from about 1 to about 12 gms per liter of solution of an agent consisting essentially of,

wherein n equals 6 or 7, said agent having been prepared by the ethoxylation of a or ,8 naphthol with said n mols of ehtylene oxide and thereafter, at a temperature within the range 50C 90C, sulfonating the resultant product with sulfuric acid, the concentration of which is at least about 90 percent.

2. The method of claim 1, wherein a naphthol is ethoxylated with 6 mols of ethylene oxide.

3. The method of claim 2, wherein said sulfonic acid concentration is about 92 98 percent and said temperature is about 80 85C.

4. The method of claim 2, wherein about 3 to 5 gms per liter of said agent is employed in said plating bath. 

2. The method of claim 1, wherein Alpha naphthol is ethoxylated with 6 mols of ehtylene oxide.
 3. The method of claim 2, wherein said sulfOnic acid concentration is about 92 - 98 percent and said temperature is about 80 *- 85*C.
 4. The method of claim 2, wherein about 3 to 5 gms per liter of said agent is employed in said plating bath. 